The invention relates generally to electrical machines of synchronous and asynchronous designs, and particularly to a rotor position estimator for a permanent magnet motor.
Interior permanent Magnet (IPM) synchronous motors have attracted attention in recent years, and are used in many industrial applications. For example, IPM machines are employed for powering high torque applications such as elevators, cranes, and machine tools. Furthermore, such machines may also be employed for conventional loads such as continuous-duty fans, pumps, and compressors. Typically, rotor position and speed information is required for motion and field-oriented control of permanent magnet motors.
Some synchronous motor drive systems employ position sensors such as resolvers and Hall effect sensors coupled to a rotor shaft of the motor to provide such information. However, these sensors add cost and weight, and may reduce the reliability of such systems. Further, use of such sensors may also result in increased maintenance requirements and need of shaft extension and mounting arrangements.
Certain other systems derive the position of the rotor based upon magnetic saliency of the rotor using an injected signal at a high frequency. Typically, such systems employ digital filters to process output signals generated in response to the injected signal. However, this may result in position errors due to undesirable propagation delays of the filters, thereby providing a time-delayed estimation of the rotor position.
In certain systems, a predefined torque related lookup table is utilized to provide a position correction term to compensate for the error in the estimated position. However, this compensation technique may require the designers to generate a lookup table that relates position error to the load current, and may require calibration based upon motor characteristics. Moreover, such techniques may result in a complex and inaccurate compensation for the position estimation error.
Accordingly, it would be desirable to develop a technique to provide an accurate estimation of rotor position in a sensorless motor drive system that uses high frequency injection for position estimation.